There has been a deep interest in the environmental causes of childhood cancer but it is difficult to study because cancer in children is rare and because it is difficult to identify past exposure levels, particularly during important periods such as pregnancy. There is now a unique opportunity to prospectively study whether maternal folic acid supplementation affects leukemia risk in children. This data analysis project leverages on the resource of the International Childhood Cancer Cohort Consortium (I4C), a large consortium of several prospective mother-child cohorts. For this analysis, the plan is to pool data from five large prospective cohorts within the I4C. These five prospective cohorts have a combined sample size of 475,353 live births from 502,466 mothers. Approximately 200 cases of childhood leukemia and 550 cases of all cancers combined have been identified as of September 2011. Data is available on maternal folic acid supplementation as well as data to assess confounding. The goal of the proposed analysis is to test the hypothesis that folic acid supplementation in pregnant mothers reduces the risk of leukemia (acute lymphoblastic leukemia and other subtypes) in their children.